chore(web): SMP prediction unit tests

> Uh-oh. Just ran a test with an SMP model; SMP predictions seem to break on iOS with these changes. I'm fairly certain part of it is due to naive use of the `String.split` method b/c surrogates, but I'm not sure whether or not that's the only reason.
>
> _Originally posted by @jahorton in https://github.com/keymanapp/keyman/issues/4411#issuecomment-775664583_

While the PR-bound issue was resolved before that PR merged, we're kind of lucky that I managed to catch it; I only caught that one because I was testing a descendant branch for something else.

Unfortunately... the test model I used isn't currently published; it's based on something a user submitted as part of a bug report. We'll want something more freely-distributable for use in a unit test.